Btw, would it not theoretically be possible to output digital audio directly to 1 channel of the jack connector ? In a similar way that most motherboard soundcards usually have a option to switch one jack output to a spdif output?

looks like you hit dead end with your AV Receiver, at least for now
you have few options
- wait for future development for USB DAC support (no costs but ETA unknown)
- use analog 3.5 mm out (no good quality and no DD or DTS)
- sell your AV Receiver and buy some with full HDMI support (involved investments)

Recently I started thread on xbmc forum http://forum.xbmc.org/showthread.php?tid=165552 asking for ALSA I2S driver support.
It seems to be a complex task to get it work on "semi-open" RPi platform, but for me it would be cool feature even without audio/video sync.

Recently I started thread on xbmc forum http://forum.xbmc.org/showthread.php?tid=165552 asking for ALSA I2S driver support.
It seems to be a complex task to get it work on "semi-open" RPi platform, but for me it would be cool feature even without audio/video sync.